Fly ash bricks
Heyl Patterson

The Characteristics of Filtered Fly Ash

Fly ash bricks

Two main byproducts of coal-fired power plants are coal bottom ash, which settles at the bottom of the boilers, and coal fly ash. Compositionally, these two types of ash have very different physical characteristics, with bottom ash being heavier and coarser while fly ash is lighter and finer. While both derivatives of coal plants have advantageous purposes, the structure of fly ash particles makes them more useful for a wider array of applications.

Composed of minuscule particles, fly ash is carried up along with hot gases through a coal power plant’s smokestack when it burns coal. Fly ash composition is such that it remains suspended in the hot, rising air, where baghouses, cyclones, electrostatic precipitators, or other mechanical dust collection devices gather it up. Fly ash can be used for numerous purposes, consisting of alumina, iron oxide, silica, and trace elements.

Coal Fly Ash Composition and Origins

When coal is pulverized and burned in coal-fired boilers, a powdery material with fine grains is produced and carried upwards with the flue gas. Called coal fly ash, it makes up as much as 80 percent of all the ash produced within a coal furnace. Electric companies use three different types of coal-fired boiler furnaces.

These are referred to as:

  • Dry-bottom boilers: The most common coal-burning furnace type, around 80 percent of the ash produced by the plant leaves with the flue gas as airborne fly ash.
  • Cyclone furnaces: These produce the least amount of coal fly ash, composition of which is between 70 to 80 percent fly ash and 20 to 30 percent molten bottom ash.
  • Wet-bottom boilers: Also known as a slag-tap furnace, these produce less fly ash, with only about half leaving the plant along with the flue gas, while the rest remains in the furnace.

When it comes to coal fly ash, dry-bottom boilers will produce the most as a ratio of total ash produced.

Importance of Collecting Coal Fly Ash 

Generally, coal fly ash composition consists of various elements, which depend on the exact type of coal being burned and how it’s handled. Fly ash is also made up mostly of particles smaller than 100 microns. That means most fly ash particles are less than a tenth of a millimeter, which is about the average thickness of a sheet of copy paper or a strand of human hair.

The prime purpose of collecting these particulates is the potential hazard they pose. In fact, the US Environmental Protection Agency (EPA) has found that contaminants like arsenic, cadmium, and mercury are often present in coal fly ash. Composition that includes such toxic ingredients has been shown to pollute the air, groundwater, drinking water, and even waterways without proper management. Exposure to fly ash has been shown to increase rates of cancers, cognitive birth defects, respiratory infections, and other negative health effects.

There’s an additional difficulty in conveying and transporting coal fly ash. Compositional elements within it can prove toxic, threatening the health and safety of those who work with the fly ash. Bulk material handlers must thus take special care to control ash dust to comply with the many rules and regulations surrounding its collection in coal plants. Yet coal fly ash can also provide numerous benefits.

Benefits of utilizing fly coal ash include:

  • Decreasing need to put waste in landfills.
  • Increasing durability, strength and workability of materials into which it’s incorporated.
  • Lessening need for other materials.
  • Lowering costs of disposing of coal ash.
  • Reducing carbon dioxide and other greenhouse gas emissions.
  • Savings gleaned due to its use rather than that of more expensive materials.
  • Selling brings increased revenue.

About half of the coal fly ash collected from coal plants is used to produce materials like asphalt, concrete, and grout for use on roads, streets, and highways. Yet ample amounts of fly ash also get released into the environment, whether during transport, in landfills, or even in small quantities that aren’t captured at the source.

Environmental Impact of Fly Ash

Since it’s considered a hazardous byproduct from the burning of coal, fly ash compositionally poses a potential environmental impact. It presents a danger to wildlife and humans that are exposed to it as well. For this reason, the EPA regulates its use, allowing only certain companies that provide appropriate training to utilize it commercially.

Fly Ash Applications 

While it has several commercial and industrial uses, concrete mixes for the construction industry are a key purchaser of coal fly ash. Compositionally, cement containing fly ash is more durable and workable than conventional cement while also providing a more affordable option for concrete manufacturing companies. By substituting about a quarter to a third of cement within concrete, construction companies can also greatly decrease the amount of carbon dioxide they emit into the environment. Building materials like plaster, concrete piping, ceramic tiling, and bricks can also utilize coal fly ash, with substitution ratios of as much as 1.5 parts fly ash to 1 part conventional cement mix. Fly ash is also used in adhesives, composite materials, and paint.


Because of coal fly ash’s composition, it makes an excellent additive for soil, providing essential nutrients for plant growth and enhancing water retention. While improving plant growth, it can also help balance pH levels and augment water flow through clay and sandy soils. Additionally, soil amended with fly ash helps trap carbon dioxide.

Treating Wastewater

Coal fly ash composition and permeability make it practical for treating wastewater. The geopolymers contained in fly ash respond like magnets to many waterborne pollutants, including fluoride and organic compounds. When bound to the porous fly ash, the contaminant-laden particles sink, making them easier to remove.

High-End Electronics 

Another unique aspect of coal fly ash composition is the rare earth elements found within the ash. Used for complex electronics in wind turbines, smartphones, and electric vehicles, these rare earths are expensive to extract via traditional mining techniques. These elements can be removed from the ash without the need for mining by flash-heating it at very high temperatures.

Equipment for Collecting Coal Fly Ash Safely

Because of coal fly ash’s composition, it’s become a valuable resource rather than a waste byproduct from coal-based energy production. Yet, to harness its properties, fly ash must be properly processed, which involves proper drying and conditioning. This is where specialized and innovative solutions from Heyl Patterson Thermal Processing come into play. Heyl Patterson offers energy-efficient material-handling equipment for drying, calcining, and cooling processes.

Heyl Patterson’s coal fly ash processing solutions include: 

  • Dryers and coolers are designed to provide efficient removal of surface moisture and have the capability to handle the wettest of ground moisture. Heyl Patterson dryers can also complete the primary classification of fly ash sizing to reduce the burden on downstream processing. Our equipment can handle large volumes and can even be used for processing coal fly ash recovered from landfills and reclamation ponds.
  • Rotary calciners act as de-carbonizers to improve the quality of the fly ash so the material containing unburnt combustion products can be used as an additive for cement. This higher-temperature process uses indirect heating technology to minimize the amount of exhaust gases discharged from the equipment. 
  • Coolers are designed to reduce the calcined ash’s temperature to acceptable storage or transportation levels.

The beneficiation of coal fly ash makes coal-based energy production more sustainable and cleaner. At Heyl Patterson, we are taking proactive steps to protect the environment, making our world a cleaner and greener place for future generations. As such, we take our responsibility regarding our planet seriously, custom-engineering our equipment to meet your specific needs.

To learn more about Heyl Patterson’s fly ash processing solutions, contact our experts today.